Switching/depolarizing power supply for a radiation imaging device

ABSTRACT

A high voltage switching power supply ( 10 ) for an X-ray/Gamma ray imaging camera provides high voltage switching and depolarization capabilities. The power supply includes a high voltage polarity switching and an image detector charge bleeding circuit ( 90 ) and is particularly useful with high energy radiation imaging cameras utilizing Cd—Te based detector substrates, especially substrates with blocked contacts, where charge accumulation in the detector material reduces imaging efficiency.

The present application claims the benefit of prior filed U.S.Provisional Application Ser. No. 60/433,457 filed 13 Dec. 2003, to whichthe present application is a regular U.S. national patent application.

FIELD OF THE INVENTION

The present invention relates to the field of digital imaging of X-rayand gamma ray radiation. More specifically it relates to switching-typevoltage power supplies for digital X-ray and gamma ray imaging devices.

BACKGROUND OF THE INVENTION

The present invention relates to a Dynamic Imaging Camera (DIC) orScanning Camera (SCAN) having Cadmium-Telluride (Cd—Te, includingCd—Zn—Te) based radiation detectors for imaging X-ray signals. Cd—Tebased detectors for imaging X-ray signals are known in the art, and areparticularly useful for high energy radiation imaging. High energyradiation imaging include X-ray and gamma ray radiation in the 1.0 KeVand greater range. Typically, these detectors have a blocking contact onone face and pixel contacts on another face. In turn, the pixel contactsare bump-bonded to charge integrating application specific integratedcircuits (ASICs). The blocking contact serves to reduce the dark orambient current of the detector by a factor of 3 to 10. Lower darkcurrents mean higher sensitivity to incoming x-rays (signal).

However, a problem can exist in a Cd—Te detector having a blockingcontact. Such contacts (e.g., Indium based blocking contacts) canpolarize after a few seconds of operation, i.e., one to several hundredsof seconds. Polarization means that the detector starts to loose signaland the image loses definition or acuity and gets more blurry.Polarization happens due to gradual electric charge trapping inside thematerial bulk of the Cd—Te detector. Previously, one could not use aCd—Te detector material in this X-ray imaging mode for more than a fewseconds, due to the polarization effect.

Therefore, it would be useful in the field to have a means forpreventing or overcoming the effect of electric charge trapping in highenergy X-ray imaging systems utilizing Cd—Te based radiation detectors.

SUMMARY OF THE INVENTION

A Cadmium-Telluride (Cd—Te) based DIC detector typically requires a highvoltage (HV) bias potential to operate properly. Unfortunately, suchdetectors can quickly accumulate an electrical charge and becomepolarized. The polarization charge offsets the HV bias potential andadversely affects operation and image quality of the camera imagingdevice. Once it becomes polarized, the detector unit requires a“refresh” action, i.e., the bleeding-off of the trapped or accumulatedelectric charge to depolarize the detector unit and restore operationalefficiency of the device.

The present invention is a high voltage (HV) switching power supply foruse with a high energy X-ray camera imaging device with a Cd—Te baseddetector (including a CdZnTe based detector). Such X-ray imaging camerastypically comprise a detector substrate bonded to a CMOS substrate andmounted to an interface/signal processing board, in combination with apower supply. The output from the camera is typically communicated to acomputer for image processing. The depolarizing, switching HV powersupply of the present invention is intended as a power supply for suchan X-ray imaging camera.

The present HV switching power supply enables the use of Cd—Te detectors(especially those having blocking contacts) in dynamic imaging cameraX-ray imaging systems and scanning camera/sensor imaging systems. The HVpower supply is used to supply HV to the Cd—Te detector of the X-rayimaging system. The HV output of the HV power supply is switchable(on/off) at user defined intervals. For example, every few seconds theHV output of the switching power supply automatically cycles off for afew milliseconds and then very fast on again. When the HV output goesoff, any electrical charge trapped at the detector(s) is able tobleed-off, which reverses or prevents polarization of the detector. Thisprevents the accumulation of electric charge and polarization of a Cd—Tedetector having a blocking contact. The prevention of polarizationallows continuous usage of a Cd—Te type detector DIC imaging device, andenables the use of such devices for inline imaging, e.g., innon-destructive testing or automated X-ray inspection systems.

The present depolarizing, HV switching power supply provides both highand low voltages useful for powering a Cd—Te base radiation detector,with or without a blocking contact. Typical low and high voltagerequirements for Cd—Te base radiation detectors are known in the art.For example, low bias voltage requirements for the Cd—Te type detectorsare on the order of +/−1.0V to +/−15.0V DC to operate the detector'sinternal circuits. The present HV switching power supply also providesan adjustable high bias voltage from +80 VDC to +450 VDC for driving thedetector.

By using the present power supply that switches on/off the High Voltageas described herein, a plurality of dynamic imaging applicationsutilizing CdTe or CdZnTe detectors becomes possible. The applicant hasdeveloped already cameras that operate at 50 frames per second, 100 fpsor even 400 fps. These cameras operate smoothly over many hours orindeed days without a need to manually refresh the detectors (bymanually powering off, waiting and then switching the HV on again). Thesmooth, stable and uninterrupted operation in X-ray imaging applicationsis essential. Example applications where such DIC or SCAN cameras can beused included but are not limited to non destructive testing, inlineinspection, automatic X-ray inspection, dental panoramic imaging,Computerized Tomography etc.

Additionally, while it was emphasized that the current invention ismostly suitable for CdTe or CdZnTe based detectors with a blockingcontact, it can also have application in CdTe or CdZnTe detectorswithout blocking contact, but equipped with Platinum (Pt), Gold (Au) orother conventional contacts. Even such conventional contacts can createpolarization after many minutes or hours and a power HV supply asdescribed herein is ideal for the smooth and stable operation over manyhours or indeed days.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 are schematic representations illustrating the presentdepolarizing/switching power supply and component circuits.

FIG. 6 (rising slope) displays the ramp-up and ramp-down waveforms of anactual Cd—Te semiconductor detector substrate where condition of thepresent HV power supply were: Ch1: HV_EN=5V, 50 μsec/div.; Ch2: DetectorVoltage, 100 v/div.

FIG. 7 (falling slope) displays the ramp-up and ramp-down waveforms ofan actual Cd—Te semiconductor detector substrate where condition of thepresent HV power supply were: Ch1: HV_EN=5V, 50 μsec/div.; Ch2: DetectorVoltage, 100 v/div.

FIG. 8 (rising slope) demonstrates the flat slope produced by a purecapacitive load and the power supply current limiter, where: Ch1=HV_EN,and Ch2=Capacitor Voltage, 100 μsec, 100 v/Div.

FIG. 9 (falling slope) demonstrates the flat slope produced by a purecapacitive load and the power supply current limiter, where: Ch1=HV_EN,and Ch2=Capacitor Voltage, 100 μsec, 100 v/Div.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, the details of preferred embodiments ofthe present invention are graphically and schematically illustrated.Like elements in the drawings are represented by like numbers, and anysimilar elements are represented by like numbers with a different lowercase letter suffix.

FIG. 1 generally illustrates the present depolarizing/switching powersupply 10 of the present invention. In a preferred embodimentexemplified in FIG. 1, the present invention was externally suppliedwith +24V DC from a medical grade external power source (not shown). Ofcourse, an alternative such a power source of an appropriate supplyvoltage is known to and selectable by one of ordinary skill in the artfor practice in the present invention, either internally or externally.In this embodiment, the supply power requirement for the presentswitching power supply was on the order of about 10 W.

As illustrated in FIG. 1, the present HV switching power supply 10comprised the combination of five main circuits: a control/conditioningcircuit 18, an internal power supply 30, a low voltage power supply 50,a high voltage power supply 70, and a high voltage switch 90. Theconditioning/control circuit 18 has an external power source connection20 and included a main power on/off switch 24. The conditioning circuit18 conditions the external power appropriately for use by the othercircuits of the power supply 10. A main operating voltage powerconnection 22 communicates the conditioned electrical power to the othercircuits of the power supply 10. Preferably, the present HV switchingpower supply 10 is electrically shielded, as is accomplishable by one ofordinary skill in the art. In a preferred embodiment, the switchingpower supply was housed in a metal casing (not shown), which casing wasconnected to a power supply ground J2-1 to attenuate or eliminateelectromagnetic interference (EMI). Generally, the connectors utilizedin the power supply are protected from electrostatic discharge (ESD) andfiltered for EMI, and the DC voltage connections are protected fromreverse polarity and voltage spikes. Additionally, the high voltageenable signal circuit is protected from over voltages and has apull-down feature. The know-how to accomplish such protections in thepresent invention are known to and practicable by the ordinary skilledartisan. Preferably, shielded cable is utilized for current carryingconductors and coaxial cable for bias voltage conductors.

FIG. 2 is a schematic diagram of a preferred embodiment of an internalpower supply circuit (int_supply) 30 practiced in the present HVswitching power supply 10: The internal power supply 30 provides lowvoltage power as required for the other circuits of the presentswitching power supply 10. The internal power supply 30 comprises alinear regulator (N5) 32 and linear regulator circuit 34 adjusted toprovide a +12V DC output 46. An LM317 integrated circuit was utilized asthe linear regulator 32 of the linear regulator circuit 34. The internalpower supply 30 also comprises a switching regulator (N1) 38 andswitching regulator circuit 40. The switching regulator circuit 40provide the low negative voltage (e.g., −5V DC in the embodimentillustrated) as required in the other circuits of the HV switching powersupply 10 at the negative bias voltage output 42. A National LM2672integrated circuit 260 kHz switching regulator in buck-boostconfiguration was utilized as the switching regulator 38. A minimumdrain for the negative bias voltage output 42 was provided by an LED 44.

FIG. 3 is a schematic diagram of a preferred embodiment of a low DCvoltage power supply circuit (power_supply) 50 practiced in the presentHV switching power supply 10. The power supply circuit 50 provides lowDC voltages for operating external devices, like the detector unit(s),connected to the power supply 10. In the exemplified preferredembodiment, the power supply circuit 50 comprised two switchingregulator circuits 52 & 53 to provide +6.7V and +5V respectively tooperate detector elements. National LM2672s regulators 56 were used inthe switching regulator circuits 52 & 53 to respectively provide a +6.7v output (Vout1) 58 and a +5V output (Vout2) 60. The LM2672s regulators56 of both switching regulator circuits 52 & 53 were disposed in buckconfiguration. In this preferred embodiment, the +5V switching regulatorcircuit 53 had a green SMD LED 62 and the +6.7V switching regulatorcircuit 52 had a yellow angled LED 63 to differentiate and indicate thelow voltage power supply 50 was turned on.

FIG. 4 is a schematic diagram of a preferred embodiment of a highvoltage, switching power supply circuit (HV_supply) 70 of the presentdepolarizing/switching power supply 10. The HV power supply circuit 70provides a high voltage bias output 72 to the high voltage switchcircuit 90 (see FIG. 5). In the preferred embodiment shown, the HVswitching power supply circuit 70 used a standard UC3842 current-modePWM controller 76 operating in a boost configuration at about 75 kHz. Avoltage feedback loop was used to adjust the voltage at high voltagebias output 72 to the desired level (about +350V in the presentembodiment). The current feedback protected the switching power supplycircuit 70 from short circuits and provided good transient response,improving HV bias rise time.

The voltage division utilized in the voltage feedback loop of the HVpower supply circuit 70 was heavy and yielded a ripple voltage of 1–2Vpp without compensation. A compensation circuit synchronized to theUC3842's oscillator circuit added an artificial ramp onto each currentpulse. The compensation ensures the power supply did not skip pulses,and limited the voltage ripple to about 200 mVpp. A constant currentload was used to provide about a 1.0 mA current drain for the powersupply independent of the output voltage.

FIG. 5 is a schematic diagram of a preferred embodiment of a highvoltage switching circuit (HV_switch) 90 practicable in the present HVdepolarizing power supply 10. As shown in FIG. 5, the HV switchingcircuit 90 provides a high bias voltage at its HV voltage output 92 inresponse to the presence of an active high voltage enable signal at itssignal input (HV-EN) 94.

However, when the high voltage enable signal at the signal input 94 isinactive or disabled, the HV switching circuit 90 provides a −5V DC biasat its voltage output 92. Additionally, when the high voltage enablesignal at the signal input 94 goes inactive or is disabled, an FETsub-circuit in the H_switch 90 is cuts off the high voltage biasvoltage. When the high voltage bias voltage is cutoff, the bias voltageat the HV voltage output 92 is pulled down to −5V, causing a reversal ofthe biasing current in the Cadmium-Telluride photo-conductor material.Reversing the biasing current in the photo-conductor material bleeds offthe trapped electrical charge and de-polarizes the detector unit.

The Q5 FET 100 is connected in series with the HV bias. It has a pull-upto the HV bias, resulting in an output of HV-Vgs in the steady-state.During the ramp-up, current flowing through resistor 102 causes avoltage differential. The Q7 transistor 104 pulls the FET gate 100closed when the set current limit is exceeded. This results in atriangular waveform for the HV bias voltage.

The Q3 FET 106 pulls the Q5 FET gate 100 down to −5V when open. Thiscloses Q5 FET 100 and reverses the bias voltage. Q3 FET 106 has asimilar current-limiter circuit as Q5 FET 100, resulting in a lineardown slope. Opening and closing Q3 FET 106 enables controlling the biasvoltage. The voltage of the Q3 FET gate 106 is controlled by the highvoltage enable signal at the signal input 94. When the high voltageenable signal at the signal input 94 is active (enabled or “pulledhigh”), an indicator LED 110 was lit.

The high voltage switching circuit 90 can be operated at a much higherswitching frequency than in the illustrated embodiment. There is about a50 μsec initial delay between low-to-high bias voltage transition andthe beginning of the HV bias voltage ramp. The delay is likely caused bythe large resistors used in pull-up circuit and the FET gatecapacitance. Modification of the characteristics of these componentscould lessen the delay. It was intended in the present embodiment thatthe imaging system operate at about 50 frames per second, thus enablingdynamic imaging, the X-raying of moving objects.

HV bias voltage rise and fall time are determined by the bias voltagecurrent limiter (see FIG. 5) and the capacitance of the DIC detectorunit. Approximate state change time (t) is given by:t=(350V*C)/5 mAAbout 6–7% of the decay/growth time (t) is not current limited. Loadresistance seen by the detector capacitance (C) was not known. Anaccurate result could be measured separately for each detector and powersupply 10 combination. An approximated result has to be multiplied toestimate when the HV bias voltage has settled within 1% of steady state.See FIG. 6.

FIGS. 6–9 display the ramp-up and ramp-down waveforms of an actual Cd—Tesemiconductor detector unit. The unit under testing had four 1 cm². ADIC 100 has a 25 cm² area. The falling slope exhibits undershoot, whichappears to originate from the Cd—Te detector unit. The undershootamplitude is limited by the power supply 10 current limiter of the HVswitch 90. The illustrated signal is almost a perfect 5 mAcurrent-limited slope until the undershoot peaks.

Rise time is about 200 μsec, fall time is about 250 μsec. However, thedetector takes several milliseconds to stabilize after the state change.Therefore, the ability of the power supply to ramp the high voltage isnot the limiting factor in the depolarization process. The figuresdemonstrate the flat slope produced by a pure capacitive load and thepower supply current limiter. The undershoot exhibited by the fallingslope of the Cd—Te detector unit is not present with a capacitive load.

While the above description contains many specifics, these should not beconstrued as limitations on the scope of the invention, but rather asexemplifications of one or another preferred embodiment thereof. Manyother variations are possible, which would be obvious to one skilled inthe art. Accordingly, the scope of the invention should be determined bythe scope of the appended claims and their equivalents, and not just bythe embodiments.

1. A high energy X-ray imaging system comprising: a Cadmium-Telluridebased radiation detector unit in a high energy imaging camera; and adepolarizing, switching high voltage power supply 10 with electriccharge bleeding means, the power supply 10 in electrical communicationwith the radiation detector unit.
 2. The high energy imaging system ofclaim 1, wherein the depolarizing power supply 10 comprises: an externalpower conditioning circuit 18 connected to an external power source 20and providing a main operating voltage power connection 22 forcommunicating conditioned electrical power to other circuits of thepower supply 10; an internal power supply 30, in electricalcommunication with the operating power connection 22, providing lowpositive and negative voltage power as required for the other circuitsof the switching power supply 10; a low voltage power supply circuit 50,in electrical communication with the operating power connection 22,generates and provides low DC voltages for operating the detector unit;a high voltage power supply 70, in electrical communication with theoperating power connection 22, generating and providing a high biasvoltage at a high voltage bias output 72 for communication to the highvoltage switch circuit 90; and a high voltage switch 90, in electricalcommunication with the operating power connection 22, the high voltageswitch 90 being a fast-on/fast-off switch, and alternately providing ahigh DC voltage bias and low opposite polarity DC bleed voltage at a HVswitch output 92 in response to the presence of an appropriate signal ata signal input 94, wherein the low opposite polarity DC bleed voltage atthe HV switch output 92 comprises the bleed means for depolarizing thedetector unit.
 3. The high energy imaging system of claim 2, wherein thehigh voltage switch 90 alternately provides a high positive DC voltagebias and low negative DC bleed voltage at the HV switch output 92 inresponse to the presence of a signal at the signal input 94.